1) Goal: To determine the precise nature of damage in macromolecules caused directly by ionizing radiation and to utilize this knowledge in the application and extension of radiation target analysis. 2) Importance: Radiation target analysis is the only method which can determine in vivo the mass of molecular structures required for a specific biochemical process. 3) Recent progress: The size of several different proteins which are active in biological processes was determined by the radiation technique and reported. These include several different enzymes (phenylalanine hydroxylase, human hepatic lipase, and the transcription termination protein Rho), receptors (avermectin binding site from Drosophila) and transporters (reduced glutathione).